photoelectric counter

A device used to count objects that pass a given point by allowing each object to obstruct a beam of light falling on a photocell. Slow counting rates can be recorded by a mechanical counter, but high rates require an electronic counter such as those used in computers. Some workshop measuring devices are made to produce interference fringes falling on a slit behind which is a photocell connected to a counter. Very precise measurements are possible in this way, as the fringe separation is one wavelength of light.